The objective of this research is to delineate the role of the Na plus K ions-ATPase as a pharmacological receptor in the cardiovascular system. Because this enzyme may be the receptor for the inotropic actions of the cardiac glycosides a detailed investigation of the interaction of ouabain with the Na plus K ions-ATPase is proposed. This will examine the mechanism of ouabain binding, the relationship between the two patterns of ouabain binding and the factors which affect the stability of the ouabain-enzyme complex. To specifically label and purify the ouabain binding factor it is proposed to develop a photo-affinity label for the ouabain binding based on the strophanthidin molecule. The Na plus K ions-ATPase may also be the pharmacological receptor for ethacrynic acid. Experiments are outlined to determine the physiologically significant mechanism of the ethacrynic acid inhibition of this enzyme and to resolve the conflicts in the literature concerning its in vivo inhibition of renal ATPases. Experiments are also proposed to determine whether or not fluoride inhibition of the Na plus K ions-ATPase is involved in methoxyflurane nephropathy. The mechanism of the Na plus K ions-ATPase will also be investigated in detail, particularly the mechanism of the calcium ion inhibition of this enzyme system and that of the K ion induced conformational change of the ouabain-enzyme complex. The latter phenomenon results in the stabilization of such a complex and will be studied by the spin labeling of the enzyme with an NEM-derivative or glycoside-derivative as well as by monitoring the stability of the ouabain-enzyme complex under various conditions.